Editor's Note: This is the third in a series of five features on carbon capture and storage, running daily from April 6 to April 10, 2009.

For more than a decade, Norwegian oil company Statoil Hydro has been stripping climate change–causing carbon dioxide (CO2) from natural gas in its Sleipner West field and burying it beneath the seabed rather than venting it into the atmosphere.

The company estimates that since 1996 it has stored more than 10 million-plus metric tons of CO2 some 3,300 feet (1,000 meters) down in the sandstone formation from which it came—and all of it has stayed put, which means storage may be the simplest part of the carbon capture and storage (CCS) challenge.

The basics of carbon dioxide storage are simple: the same Utsira sandstone formation that has stored the natural gas for millions of years can serve to trap the CO2, explains Olav Kaarstad, CCS adviser at Statoil. An 800-foot (250-meter) thick band of sandstone—porous, crumbly rock that traps the gas in the minute spaces between its particles—is covered by relatively impermeable 650-foot (200-meter) thick layer of shale and mudstone (think: hardened clay). "We aren't really much worried about the integrity of the seal and whether the CO2 will stay down there over many hundreds of years," Kaarstad says.

The company monitors its storage through periodic seismic testing, a process that is not unlike a sonogram through the earth, says hydrologist Sally Benson, director of the global climate and energy project at Stanford University. That monitoring indicates that between 1996 and this past March, the liquid CO2 has spread to occupy some three square kilometers, just 0.0001 percent of the area available for such storage.

"We're not going into a salt cavern, we're not going into an underground river. We're going into microscopic holes," explains geologist Susan Hovorka of the University of Texas at Austin, who has worked on pilot projects in the U.S. "Add it up and it's a large volume" of storage space.

How large? The U.S. Department of Energy (DoE) estimates that the U.S. alone has storage available for 3,911 billion metric tons of CO2 in the form of geologic reservoirs of permeable sandstones or deep saline aquifers, according to a 2008 DoE atlas. These reservoirs are more than enough for the 3.2 billion metric tons of CO2 emitted every year by the roughly 1,700 large industrial sources in the country. Most of that storage is near where the majority of coal in the U.S. is burned: the Midwest, Southeast and West. "There are at least 100 years of CO2 sequestration capacity and probably significantly more," Benson says.

The storage seems to be long-term as well; the sequestered CO2 doesn't just sit in the rock waiting for a chance to escape. Over decades it forms carbonate minerals with the surrounding rock, or it dissolves into the brine that shares the pore space, Hovorka notes. In fact, when she tried to pump CO2 out of her test site south of Dayton, Tex. using natural gas extraction techniques, the attempts failed completely.

According to the U.N. Intergovernmental Panel on Climate Change (IPCC), which issued a special report on CCS in 2005, a properly selected site should securely store at least 99 percent of the sequestered CO2 for more than 1,000 years. James Dooley, a senior research scientist at Pacific Northwest National Laboratory and an IPCC lead author, considers that to be a reachable goal. "If it took all that energy to shove [the CO2] into that sandstone, it's going to take a lot of energy to get it out," he notes. "Like an oil field, where we get out half or less of the original oil in place, a lot of the CO2 gets stuck in there. It's immobilized in the rock."

Encouraged by the success of the Sleipner project, Statoil recently began another CO2 injection program at the Snohvit natural gas field in the Barents Sea, despite the requirement that they build a 95-mile (150-kilometer) pipeline on the seabed to pump the CO2 to where it can be sequestered.

And since 2005, oil giant BP and its partners (including Statoil) in the In Salah gas field in Algeria have been stripping the nine billion cubic meters of natural gas produced there annually of the 10 percent carbon dioxide it contains and pumping a million metric tons of liquid CO2 back into the underlying saline aquifer through three additional wells at a cost of $100 million.

BP uses a variety of techniques, including satellite monitoring, to observe the impact of the CO2 storage (and natural gas removal). Whereas some areas sank by roughly 0.24 inch (six millimeters) as natural gas was extracted, near the CO2 injection wells the land rose by some 0.39 inch (10 millimeters), according to Gardiner Hill, manager of technology and engineering for CCS at BP's alternative energy arm.

"The gas has been down there about 20 million years so we know [the reservoir] has integrity," he says. The DoE's National Energy Technology Laboratory is also working on developing appropriate monitoring, verification and accounting technologies.

BP and Statoil are not doing these CCS projects for charity, of course. A Norwegian government tax on carbon of roughly $50 per metric ton inspired the CO2 sequestration at Sleipner and Snohvit. "It costs a fraction of the tax," Kaarstad says. "We are actually making money out of this."

Both Statoil and BP foresee more money-making CO2 storage opportunities. Hill notes that if CCS is deployed on a very large scale, society will need the expertise of the oil industry—its "100 years of understanding the subsurface," he says. "We would expect the experience we are building through this to position BP to take advantage of any future business."

"My one prediction is that this is going to be a very big industry, storing CO2 underground but transporting it, as well," Kaarstad adds. "It's not going to happen overnight, but it will probably be as big as natural gas after a few decades."

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